I've read through this book and played about three sessions with a Castoff, here is my honest review:

First of all, the lore of this supplement is awesome, I love the references to real world folklores and mythologies, lots of fun research rabbit holes to jump down. The contrast between gloamspun, who usually choose their half-fae status, and eventide, who don't and are caught between worlds from birth,  is so much fun to explore in-game and makes for great roleplaying. The backstory potential here is fun to write as well. I love playing my Castoff and my next character will be an Eventide for sure. 

I do have a big complaint about this book though, it's the reason I knocked off two stars. Since this is an alternative to making Gloamspun characters, I really wish this book came with its own character sheet. A lot of my character sheet is stuff for Gloamspun that I had to cross out because it isn't relevant to my character, like Shroud, House, Principles, and Motif. It made character creation really difficult as a first-time TGD player. I also don't have a place to put things like Inhuman Appearance or Legacy, or a built-in way to differentiate my character's permanent weaknesses from the ones they only have with their Inhuman Appearance.  

Overall, I'd say this is a great supplement for people who have played TTRPGs and The Gloaming Diaries before, but I wouldn't recommend it for a first-time player or for a one-shot.